The print quality in thunderbird improved by making some changes in its font settings.
1. In Preferences, in the Display option, instead of default 'Fixed Width Font', the 'Variable Width Font' was selected. This gave a better shape to the font look.
2. In Preferences, in the Fonts option, the 'Proportional' and 'Monospace' fonts text boxes were given the option 'Sans Serif'. This also improved the look of the messages in the window too.
The message printouts are quite decent now.
Regards,
Rony.